Chapter II: The Classification of Stellar Spectra

Various methods of classification may be applied to the stars. The most obvious and the most antique is that of relative brightness. From of old the stars have been collected into ranks by “magnitude.” Or the amount of their “proper motions” may be taken as the principle of distinction. Within certain limits this is practicable, and for certain purposes it is useful. But apparent lustre and projected movement alike depend in part upon distance; they include an extraneous element; and astrophysical science considers the heavenly bodies in themselves, without regard to their spatial relations. Hence an absolute quality must be made the basis of their arrangement, and it is found in the _kind_ of light emanating from them. This system is of far more than conventional value. It affords the only clue within reach to the intricacies of stellar constitution. Schemes of spectral classification may be amended and altered; but in one form or another they are indispensable to progress.

They naturally tend to become more complex as facts multiply, and finer shades of difference are rendered manifest; yet Father Secchi’s four “types” continue fundamental. It is well, then, to keep their characteristics steadily in mind. The first is marked by strong hydrogen absorption. It consists of radiantly white stars. The second by innumerable fine metallic rulings; the sun is an example. The third type includes red stars with banded spectra like Antares, the bands being sharply terminated towards the violet, diffuse towards the red. The fourth is composed of deeply-tinted, mostly faint objects, showing wide bands facing redward, due to carbon absorption. These four groups form irremovable landmarks; but beside and between them many subordinate divisions have been set up. Vogel, Huggins, McClean have all modified, while broadly adopting Secchi’s “law of order.” They have, moreover, regarded it, not as a mere empirical formula, but as prescribed by the necessary conditions of development. Modes of classifying the stars have come to be equivalent to theories of their evolution. With this aspect of the matter, however, we are not just now concerned. The aim of the present chapter is to establish convenient distinctions without regard to their essential meanings. The unravelment of these will be attempted later on.

Miss Maury’s arrangement of the stars[330] is designed for a life-history as well. It is the most elaborate yet put forward. Based on the examination of some 4800 spectrographs taken at Harvard College, it embraces 681 objects, disposed in a progressive series of twenty-two groups, most of which are further comprised within three collateral divisions, established to meet the visible necessity for a secondary characterisation. The work is a monument of industry and skill, and will long hold a place of standard authority; but the fine gradations it emphasises, although worth putting on record, are scarcely suitable for committing to memory. Our object here is to present large outlines, leaving minute shades of difference to be dealt with as occasion may arise. There is danger of stellar classification degenerating into a maze of provisional distinctions. The best remedy is to fix attention on the summit-ranges of the landscape; when they are clearly imprinted on the mind, mastery of detail can be safely and readily acquired.

A framework of eight compartments accommodates practically all the stars. The separation is easy and natural, and the “notes” of the various classes present themselves unmistakably. The constituents of the four first show absorption spectra only; those of the four last are marked by emission as well as by absorption. No hypothesis of growth or affinity is implied by the order of succession given to the bright-line objects; only the interests of clearness have been consulted in its choice. We will now briefly describe these stellar families.

PLATE IX.

Stellar Spectra photographed by Sir William and Lady Huggins.

Fig. I. Vega (α Lyræ). Fig. II. Arcturus (Fe and Ca for comparison).
Fig. III. Rigel. Fig. IV. β Cygni, Blue Star; β Cygni, Yellow Star
(solar comparison-spectrum).
]

Class i.—_Helium Stars._—In the spectra of these brilliantly white stars, absorption by hydrogen and helium predominates. The complete “Huggins series” is stamped upon them, from the fundamental C to its “head” in the ultra-violet;[331] and at least twenty-six of the strongest helium lines, culled impartially from all the six series, show conspicuously besides. In a few helium stars, suspected of nebular relationships, the “Pickering series” of hydrogen is represented, while others betray the action of oxygen, nitrogen, and silicon. Metallic lines are faint and scarce; those identified belong to sodium, iron, calcium, and magnesium. Especially remarkable is the comparative prominence of the magnesium line λ 4481, to the exclusion of the triplet _b_, which takes the lead in the solar spectrum. The substitution, according to many authorities, indicates enormous heat. Scheiner’s criterion for high temperature is precisely the development of λ 4481, and Professor Keeler remarked that the effacement of _b_ marked a stage of heat beyond the possibility of artificial production.[332]

In these stars there is almost no general absorption; their photospheres are _unveiled_. Moreover, they seemingly possess reversing layers of very simple composition, to which circumstance their display of helium may, with much probability, be attributed. Originally included in Secchi’s first type, they were separated from it by Vogel in 1895, on the identification of their distinctive lines with those of terrestrial helium; and their importance in the sidereal scheme was accentuated by McClean’s spectrographic researches in the southern hemisphere. The lucid orbs of Orion and the swarming Pleiades are leading members of the class, which are also thickly disseminated in the Southern Cross, the Centaur, and the Greater Dog. Miss Maury’s first six groups are subdivisions of helium stars, arranged in the assumed order of their development from a nebulous condition.

We are indebted to Sir William and Lady Huggins for permission to reproduce their admirable spectrograph of Rigel, the premier helium star (Plate IX. Fig. III.), and to Sir David Gill for that of ε Canis Majoris (Plate X. Fig. 1), taken by his assistant, Mr. Lunt, who detected many lines of silicon and oxygen, in addition to strong helium-absorption, in the spectrum of this star.

Class ii.—_Hydrogen Stars._—These stars are distinguished by intense hydrogen-absorption of the ordinary kind, no Pickering lines being present. Helium-influence on their light is null, or barely perceptible. The “H” and “K” of calcium are thin but distinct. Feeble iron lines can be numerously discerned. General absorption is slight; the ultra-violet end of the spectrum lies open, imparting to hydrogen stars a bluish-white colour. Vega is a perfect example; its spectrum, photographed by Sir William and Lady Huggins, is shown in Plate IX. Fig. I. The black band to the right is the fifth line of hydrogen (Hε). It masks the calcium “H”; but “K” appears well to the left.

Hydrogen—sometimes called “Sirian” stars—abound in the heavens. They form the main part of Secchi’s first type, and are distributed by Miss Maury into five groups, numbered vii. to xi.

Class iii.—_Solar Stars._—The Fraunhofer spectrum sets a pattern copied, with slight variations, by the members of this class. Its leading feature is the powerful development of “H” and “K.” Other metallic lines are innumerable, but mostly sharp and thin. Four hydrogen lines are normally present, ultra-violet members of the series showing decisively only in stars like Procyon and Canopus, which may be regarded as intermediate between the Sirian and the solar classes. A yellow tinge corresponds in the latter to a veiling of the blue end of the spectrum, similar to that perceptible in the sun. Solar stars, then, resemble him, not only in the composition of their reversing layers, but in the possession of “smoky” envelopes. A spectrograph of Arcturus by Sir William and Lady Huggins is shown in Plate IX. Fig. II., and one by Sir David Gill of α_{2} Centauri, the brighter member of the southern binary, in Plate X. Fig. 2. The precision of its correspondence with the solar spectrum may be seen by a glance at the comparison strips in this latter figure. Stars of the solar class constitute Secchi’s second type, and are included in Miss Maury’s groups xii. to xvi.

PLATE X.

1. Spectrum of ε Canis Majoris (central strip) compared with Lines of
Hydrogen, Helium, Oxygen, and Silicon.

2. Spectrum of α Centauri (central strip) compared with Solar Spectrum
(outer strips).

_N.B._—The relative displacement is due to a difference of temperature
at the exposure-times.
]

Class iv.—_Stars with Fluted Spectra._—Two kinds of absorption are distinguishable in them. A linear system, somewhat reinforced from the Fraunhofer model, has superposed upon it a set of dusky flutings, about ten in number, of undetermined chemical origin. They suggest action by oxides, the formation of which in stellar atmospheres would seem to imply a considerable reduction of temperature. None of the bands occur in the more refrangible part of the spectrum, so that the photographic differences between solar and “fluted” stars are easily overlooked. Nor is there an abrupt transition from one class to the other. From Capella the line of connection passes unbroken through Arcturus and Aldebaran to β Andromedæ and α Orionis (Betelgeux), absorption settling down more heavily on the blue rays, and metallic lines gaining strength at the expense of the truncated hydrogen series, until the fluted type is definitively formed. It is equivalent to Secchi’s Type iii. Within its compass a progression of objects with deepening bands, such as Miss Maury has arranged in her Groups xvii., xviii., and xix., can readily be followed out. It may be said to terminate with α Herculis, a star of the third magnitude, displaying magnificent prismatic chiaroscuro. A drawing of its spectrum by Mr. Espin is copied in Plate XI. Fig. 1.

Fundamentally, the same series of bands recurs in all the individuals of this class. They vary from star to star both in relative and in absolute intensity, but their identity remains unmistakable. The presence of certain determinate atmospheric ingredients fixes the type, and no others can replace them. The stars belonging to it are in diverse degrees red or orange, their blue emissions being largely arrested in the precincts of their photospheres. They must hence be intrinsically brilliant far beyond the proportion of their visual, and, still more, of their photographic magnitudes. Their apparent lustre, that is to say, is small compared with the masses that may reasonably be assigned to them. Their light is markedly unstable, and many are subject to periodical variations of exceedingly wide range. These give bright-line spectra of a very definite character, which it seems advisable to set in a class apart.

Stars with fluted spectra (conveniently designated as “Antarian,”[333] from their exemplar, the _lucida_ of the Scorpion), although rare compared with Sirian and solar stars, are found plentifully in every part of the sky. No comprehensive catalogue of them exists; the number of those already known might, however, be roughly estimated at a couple of thousand. They must be vastly remote. None have sensible parallaxes, and very few show appreciable proper motions.

Class v.—_Carbon Stars._—These have also banded spectra, but of a totally different stamp. Three shadings, particularly conspicuous in them, testify to strong absorption by carbon vapour. Vogel found them to reverse exactly the spectrum of an alcohol flame.[334] Scheiner conjecturally identifies the absorbent material with acetylene.[335] Others hold it to be pure carbon. There is much uncertainty on the point. The carbon-bands are of different degrees of obscurity in the various members of the class, nor do they in all preserve the same relative strength.[336] Subordinate bands, too, of unknown origin diversify these spectra more or less strikingly. But all are designed on the same pattern; it is only the mode of _printing off_ that varies. They include as well many dark lines, notably Fraunhofer’s “D” and “E,” representing absorption by sodium and iron. A characteristic dusky streak at λ 576 awaits chemical interpretation.

PLATE XI.

1. Spectrum of α Herculis. Drawn by Mr. Espin, 30th June 1894 (red end
to the right).

2. Spectrum of 152 Schjellerup (yellow section). Photographed with the
Yerkes 40-inch refractor.
]

Carbon stars glow like rubies in the sky; they are, for the most part, fiery red objects. To the eye they make a poor show. The brightest—19 Piscium—is of 5·5 magnitude; and only three in the northern and four in the southern hemisphere, out of about 250 recorded, exceed the sixth. This is not surprising when we consider that but a small percentage of their rays can escape stoppage by enfolding vapours. They would seem, besides, to be plunged in greater depths of space than Sirian or solar stars; so that they ought perhaps, allowance being made for these disadvantages, to take rank as potent light-givers. The coloured “zones,” or luminous intervals in their spectra, are, at any rate, curiously vivid and sparkling. Most are to some extent, some are to a large extent, variable. Their photographic examination—rendered arduous by the quality of their light—afforded Professor Hale in 1898 the interesting discovery, imperfectly anticipated by Secchi, of several unfamiliar bright lines superposed upon their dark shadings.[337] Hydrogen lies low in carbon stars. It exerts no visible absorption, and displays no traceable emission. Their relationships with other stellar families are obscure; connecting spectral links are not altogether wanting, but they are of secondary importance. The class formed by them is coextensive with Secchi’s Type iv. and with Miss Maury’s Group xxi. Four specimens are given in Plate XII., from spectrographs taken by Professor Hale and Mr. Ellerman with a train of three prisms fitted to the eye-end of the Yerkes forty-inch refractor. The wealth of detail shown is so great as almost to obliterate the general columnar effect.

Class vi.—_Stars with Fluted Spectra showing Bright Hydrogen Lines._—Mira is the typical star of this class. All its members (save one doubtful case) are pronounced variables. They fluctuate in colour too, but show in general a decided orange or ruddy hue. The flutings are very marked, and they tend to deepen and widen as the stars lose light. Essentially the same as in Class iv., they overlie a similar metallic line spectrum. Vivid hydrogen rays come into view with the approach of each maximum, and fade after it has passed. They seem, however, to persist much longer in some stars than in others. The series is, indeed, at all times incomplete. Its first term—the crimson C—is often missing; the second, F, is by no means invariably present; the stress of brilliancy is, in certain stars, laid upon the third, in most upon the fourth line, the fifth being always concealed by the dense, distended H of calcium. The succession of bright lines is resumed in the ultra-violet, and continued to the limit of the spectrum, which is curtailed by strong general absorption. A spectrographic impression of Mira by Father Sidgreaves is shown in Plate XIII. (1). It extends from orange to indigo, but stops short of H and K. There is no assured trace of green hydrogen, while the two blue beams are lustrous. A corresponding print of the spectrum of α Herculis appears on the same plate below that of Mira. The yellow ray of helium shines in some members of Class vi., and several dark lines in the spectrum of Mira coincide approximately with more refrangible lines of the same substance. Through the presence of emissive symptoms in fluted spectra, more than a hundred new variables have been photographically discovered by Mrs. Fleming and her staff at Harvard College. She subdivides them into eleven families, marked by the varying relative brightness of the hydrogen lines,[338] while Miss Maury includes them all in her twentieth group. Secchi’s third type likewise claimed them; its limits were indeed defined before their singularities had been noticed.

Class vii.—_Helium Stars with Bright Lines._—These objects give the characteristic dark-line “Orion” spectrum, variously emblazoned with rays of hydrogen, helium, and a few other substances. In some the bright and dark lines are ranged side by side, in others they are superposed, the system of reversal being tripled by the addition of dark _threads_ drawn across the emission rays. The historical variable “P Cygni” exemplifies the former variety, γ Cassiopeiæ the latter. The fundamental C is perhaps always the brightest line in these spectra, and there is a uniform decrease in the lustre of the hydrogen-series as it progresses upward.[339] In many cases its lower members show by emission, the rest by absorption. The bright spectrum may indeed be reduced to a solitary C. The same rule applies to helium. The circumstance, however, that the lowest terms of each series are those vivified, becomes evident only when the lines present are sorted out in their due sequential order. It is unapparent on a collective view of them.

About fifty bright-line helium stars are known, and fresh specimens are yearly swept up in the course of space-sounding operations at Harvard College. A fuller acquaintance was gained with thirty-two among the number by Professor Campbell’s scrutiny of their spectra with the great Lick refractor in 1895. Much hesitation prevails as to their proper place in systems of stellar classification. Most usually they are treated as a subdivision of the dark-line helium class, but Miss Maury and Miss Cannon leave them outside the framework of their respective schemes, appending valuable discussions of individual peculiarities.[340] There is much to be said for this mode of procedure. We are enabled, by the kindness of Father Sidgreaves, to reproduce in Plate XIV. a spectrograph of γ Cassiopeiæ taken at Stonyhurst, 7th March 1898. Among the bright lines imprinted on it are several due to magnesium,[341] namely, the _b_-triplet prominent in the sun, and the blue ray at λ 4481, specialised by Scheiner as marking a high grade of heat.

PLATE XII.

Spectra of Carbon Stars. Photographed with the 40-inch Yerkes
Refractor by Professor Hale and Mr. Ellerman.

1. 280 Schjellerup (Mag. 7·8).
2. 19 Piscium (Mag. 5·5 Var.).
3. U Hydræ (Mag. 5·5 Var.).
4. 152 Schjellerup (Mag. 5·5).
]

Class viii.—_Wolf-Rayet Stars._—Acquaintance with these objects began in 1867 with the discovery, by MM. Wolf and Rayet of the Paris Observatory, of three small stars in Cygnus, giving a spectrum composed mainly of blue and yellow effluences. Then on 24th December 1871, Respighi[342] observed the brilliant prismatic radiance of γ Argûs ( = γ Velorum), which proved to be of the same quality, although vastly superior in quantity. No other star of the kind exceeds the sixth magnitude, and over one hundred of them have been already recognised. Their distribution is remarkable; all are situated in or quite close to the Milky Way, except a considerable group located in the Magellanic Clouds;[343] and the Magellanic Clouds obviously reproduce many of the conditions of the Milky Way.

The leading spectroscopic distinction of the Wolf-Rayet class is the display of the Pickering series of hydrogen. Five of its constituent lines have been recognised, but they are not all equally bright. The upper ones may even appear dark. Emission-bands in the blue, on the other hand, never fail to be visible. Two at least are simultaneously or alternatively present. The more refrangible at λ 4688 is the azure beam identified by Rydberg with the leader-line of the otherwise unknown principal series of hydrogen. Its associate at λ 4652 may possibly owe its origin to nitrogen, but this remains to be proved. Helium lines show, both bright and dark, in these stars; in the same spectrum D_{3} occasionally gleams golden beside its dusky fellow, the noted “Orion” absorption ray at λ 4472. Similarly, in the Huggins hydrogen series, a vivid C may have for its companions an almost neutral F (Hβ), and obscure Ηγ and Ηδ.

The Wolf-Rayet spectrum is then triple. A band of continuous light, fairly strong in the ultra-violet, forms its basis. Absorption lines and bands are superposed, a few of them due to hydrogen and helium, but for the most part unclaimed by any terrestrial substance. To these are added hydrogen, helium, and anonymous bright rays in varying degrees of profusion. No metallic lines, bright or dark, have been recognised. Stars of this description are white or yellowish. They are rarely or never variable. Pickering combined them in 1891 with planetary nebulæ into a “Fifth Type of Spectra”;[344] yet, certain nebular affinities notwithstanding,[345] they lie well away on the stellar side of the dividing line between the two sidereal realms.

The eight stellar divisions just enumerated comprehend as nearly as possible all the stars spectroscopically examined up to the present. The few left outstanding are, in general, difficult objects, which have been casually or defectively observed. When better known, they will probably avow affinities not at first sight apparent. Our classification may then fairly claim to be exhaustive; it certainly rests upon broad and unmistakable distinctions. And it is no small achievement to have obtained a bird’s-eye view of the celestial “maze of error.” Order is not knowledge; _vere scire est per causas scire_; but it is an indispensable preliminary to its attainment.

PLATE XIII.

Spectra of Mira (1), December 1897, and of α Herculis (2), February
1898. Photographed by Father Sidgreaves (_Knowledge_, vol. xxi. p.
113).

1 = λ 4227. 2 = λ 4420. 3 = λ 4581. 4 = λ 4757. 5 = λ 4951. 6 = λ
5162. 7 = λ 5447. 8 = λ 5597. 9 = λ 5756.
]